There are numerous industrial processes which utilize gas- or oil-fired equipment such as furnaces, ovens, driers, boilers, heated baths, etc.; these will oftentimes be referred to herein by the term "furnace" intended to be generic to this class of heaters. This description will also refer specifically to gas-fired furnaces, because of their popularity. However, the invention is equally applicable to oil-fired equipment. Many of such furnaces employ multiple stage units requiring multiple burners. Oftentimes, they must be fired in a particular sequence. In almost all cases, they must be shut down for a flame failure malfunction in order to avoid the possibility of unwanted combustion or explosion. Control systems for these units can be complex or simple, but in most cases they have been special purpose systems which have little flexibility beyond the capabilities provided the system when it is installed and married with the furnace line.
It has been typical to utilize multiple burner controls which are of the hard-wired variety and dedicated to a specific furnace line. Part of the rationale driving that approach, it appears, is the fact that such systems are highly safety-related, and the production of single purpose devices avoids the availability of options and option switching which might impact the operating safety of the system. Thus, when a furnace line and its dedicated control system is installed, set up, tested and put into operation, it continues to monitor the assigned apparatus without intervention by an operator so that should a failure occur, it will be reliably reported, without the possibility of operator intervention having altered the system in a possibly detrimental way.
Flame sensor transducers which have been used in the past include both flame rod and ultraviolet type transducers. While each has its desirable characteristics, it is not uncommon to have systems where both types of transducers are used in the same furnace system. For example, flame rods may be used to monitor pilot flames, whereas ultraviolet transducers might be used for the main burners. The prior art has attempted to produce continuously variable or analog signals from the transducers which are indicative of the quality of the flame sensed by the transducer. Such analog signals have been brought to test points or have been brought to a selector switch so that an operator, using a voltmeter, can check the test points or manually select individual flames to read an analog voltage whose magnitude is indicative of the quality of the flame.
One of the significant events in connection with such control systems is a flame failure, and typically upon detection of a flame failure, the system is configured to go into an ordered shutdown. Prior art systems have been able to maintain a record of which flame failed and caused the shutdown, but insofar as applicants are aware, much of the information on the status of the system at the time of the failure is lost, because the status of the system clearly changes during the shutdown process. Thus, a maintenance technician may have information on which burner failed and the time of failure, but will likely have little additional information on the relationship of the failed burner to other areas of the system and their status at the time of the failure.
Due to their hardwired inflexibility, prior art control systems provided little opportunity to the operator to perform system functional tests by means other than the specific functions hardwired into the system. Thus, in order to test a particular feature, the operator would very likely have to run the system through its ordinary startup mode and simply take note of the characteristic of interest as the system automatically progressed through its hardwired inflexible startup sequence.